Mini star could offer haven for life

By Hazel Muir

YET another planetary system in the making has been discovered – but this one is a bit different from the many other extrasolar systems discovered so far. Instead of surrounding a sun-like star, it is centred on a brown dwarf that is itself barely bigger than a giant planet. And despite the host star lacking the nuclear reactions that make our sun shine, its disc could one day spawn habitable Earth-sized planets.

The discovery calls for a rethink of how many different kinds of planetary systems there might be. “We just think of planets forming around stars about the mass of our sun,” says Kevin Luhman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “But they could form in more exotic situations around very small brown dwarfs. There might be little mini solar systems out there.”

Brown dwarfs are failed stars with masses of between 15 and 70 times that of Jupiter. They are thought to form like ordinary stars, from a collapsing cloud of gas and dust. But unlike ordinary stars, they don’t generate high enough temperatures to trigger the sustained fusion of hydrogen nuclei that is the source of the tremendous amounts of energy released by larger stars.

Earlier studies have shown that several medium-sized brown dwarfs are surrounded by protoplanetary discs, but astronomers using ground-based telescopes find it hard to study the discs because they are fainter than the infrared radiation coming from the brown dwarf itself.

NASA’s Spitzer Space Telescope, launched in August 2003, gets round this problem. Spitzer is far more sensitive than ground-based telescopes to longer-wavelength infrared light, and at these wavelengths the cool disc around a brown dwarf looks ...

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